The present invention is related generally to the phenomenon of Stimulated Brillouin Scattering (hereinafter SBS) in the light guide of an optical communication system and more particularly to a method and associated apparatus for optimizing the SBS performance of such an optical communication system by using at least two phase modulation tones and identifying an advantageous operational region based on the phase modulation provided by the tones. A highly advantageous SBS contour map and its method of use are also disclosed. In addition, a modulated light producing arrangement manufactured in accordance with the teachings of the present invention is disclosed.
The phenomenon of SBS has been known in the prior art for a number of years. Essentially, SBS results when a threshold power level is exceeded within a sufficiently narrow frequency band in a fiber optic light guide. The problem of SBS has become significant with the development of lasers such as, for example, Single Longitudinal Mode lasers which readily provide an output that exceeds the SBS threshold (typically about 4 mW in, for example, a 50 kilometer fiber optic cable). Moreover, limitation of optical power to a level as low as 4 mW not only fails to utilize the output power available from state of the art lasers, but limits distance transmission through fiber optic cable by an unacceptable margin. Therefore, suppression of SBS has been contemplated in the prior art.
One effective method of overcoming the limitations imposed by SBS has been found to be the use of phase modulation. U.S. Pat. No. 4,560,246 broadly describes this technique. Indeed, the utilization of two modulation tones, specifically 2 and 6 GHz tones, to achieve the desired phase modulation is admitted prior art. In fact, as early as 1975, E. P. Ippen recognized in an article entitled Nonlinear Effects in Optical Fibers that SBS is a limitation on narrow band transmission capabilities, and low SBS threshold can, in practice, be circumvented by the use of short pulses or broad band sources. However, as will be described below, the present invention recognizes a highly advantageous and heretofore unknown method of improving SBS performance for use with at least two phase modulation sources. The method may utilize a highly advantageous phase modulation contour map for the purpose of establishing certain operating parameters of an optical communication system. In addition, a light producing arrangement is disclosed which provides for implementation of the established operating parameters.
As will be described in more detail hereinafter, a method of optimizing SBS suppression is disclosed for use in an optical communication system including a light guide for transmitting light and means for producing phase modulation of the light using at least first and second tones. In the method, an operational region of SBS suppression is established as a function of the phase modulation of the light such that the operational region identifies combinations of first and second phase modulation levels at which optimum SBS suppression is achieved for the first and second tones. Thereafter, based on the operational region, the first and second phase modulation levels are adjusted such that the system operates with optimum SBS suppression.
In one aspect of the present invention, a contour map and associated method are introduced. The contour map is especially suited for use in optimizing SBS suppression in an optical communication system in which light is phase modulated using at least first and second tones for transmission of the phase modulated light via media which exhibits an SBS threshold. The contour map includes a first axis along which the phase modulation value produced by the first tone is plotted and a second axis along which the phase modulation value produced by the second tone is plotted. An SBS suppression value is assigned to each point within a region defined by the phase modulation values produced by the first and second tones such that sub-regions of SBS suppression are identifiable with the region.
In another aspect of the present invention, one of the sub-regions identified by the foregoing contour map may be selected as the operating point of the system based on certain characteristics of the contour map at and around the location of the selected operating point. In this manner, the stability of the optical communication system may be improved, for example, in the event that the selected operating point is subject to drift.
In yet another aspect of the present invention, a light producing arrangement for injecting light into a light guide which exhibits SBS is disclosed. The arrangement includes means for generating light and means for phase modulating the generated light for the purpose of suppressing SBS. The phase modulating means includes at least first and second tone generators, having first and second adjustable output power levels, respectively, which are adjustable in increments of less than 0.5 dBm.
In still another aspect of the present invention, a tone generator is provided which includes a feedback loop so as to substantially stabilize the output of the tone generator for use in phase modulating the light output of a laser.